Share 'Just how much does adding air barrier to a knee wall insulation increase it's performance?'

I know we have a lot of scientifically minded people here and I bet someone has access to a reliable test data showing the answer to the question that's been on my mind for years:
When we add Vapor-permeable Air Barrier to those knee walls just how much exactly that increases the performance of that insulation?
I've had hard time finding results of actual test showing how effective this is.
I am…